In printing presses of the above type, turning drums are needed in order to pick up the sheet from the previous storage drum either by way of the front edge or rear edge of the sheet, at choice, and to transfer the edge thus picked up as leading edge to a subsequent impression cylinder. Transmission elements and adjusting mechanisms are needed in addition to the gripper systems to perform the sheet pick-up and sheet transfer functions and to perform the very different movement patterns. The turning drum is usually formed with spaces to receive gripper systems and transmission elements and has a cylindrical sheet support surface operative to guide the sheets, such surface extending over more than half the periphery of the turning drum. Assembly and maintenance are therefore often fairly complex. However, precisely because the movement patterns are so complex, care must be taken to ensure that the drum surface, which of course always contacts the printed side of the sheet, does not damage the previous printing and that fresh ink is not deposited on the drum and does not cause uncontrolled soiling of subsequent sheets. This is particularly important when the press is operating with sheet turning and when the materials being printed are difficult --i.e., very thin or thick papers.
DE-AS 2 604 895 discloses a turning drum, describing a device for changing over gripper movement in association with a turning drum for first printing and perfector presses. This known drum has inter alia a pit receiving pins, a torsion bar spring interconnecting the same, an eccentric pin with eccentric projections and a slotted plate. These elements are operative to clamp, mount and displace driving elements with cam rollers to drive tong grippers, such rollers being opposite cams associated with the turning drum on both sides. To close the pit interrupting the sheet-guiding surface there is a longitudinally movable bridge which also carries the pins, torsion bar spring and eccentric pin. Guiding and clamping elements must therefore be provided on the drum in order to secure the position of the bridge and to facilitate movement thereof.
In the version of this drum disclosed by DE-AS 2 547 251 the pit is closed by means of a separate filing element in order to cover the drive parts below.
In both these solutions of the problem the sheet-guiding surface of the drum is made partly of the parent material and partly from the material used for the filling element or bridge. Irregularities may therefore readily arise on the drum surface. To obviate this, the parts must be made with a relatively high degree of precision and consequently expensive production in order to fit the bridge or filling element. Also, the presence of one of the covers described means that only a reduced proportion of the transmission elements in the drum is accessible. Because the drum itself is therefore of relatively complex construction, manufacture and assembly are costly. Also, the drum surface is adaptable to printing process requirements either not at all or only at considerable cost.